Accommodation is a dioptric change in power of the eye which occurs due to a change in optical power of the lens. In a young eye, an accommodative effort results in a decrease in lens diameter, an increase in lens thickness and a steepening of lens surface curvatures. Presbyopia, the age related loss of accommodation, has been attributed to age changes in both the extralenticular (ciliary body and muscle) and lenticular (lens and capsule) accommodative apparatus. However, there are significant age changes in the crystalline lens that contribute to the progression of presbyopia. In presbyopes, an accommodative effort results in convergence, contraction of the ciliary muscle, but no corresponding accommodative change occurs in lens thickness, diameter or surface curvatures. The human crystalline lens gradually loses the ability to undergo accommodative changes with mechanical stretching and fails to undergo accommodative changes in focal length after 60 years of age. Mechanical compression tests show that the human lens undergoes an exponential increase in hardness. Other age changes in the human lens also include systematic age related changes in spherical aberration and focal length. Dynamic analysis of accommodative changes in rhesus monkey eyes shows that the crystalline lens undergoes a greater age related reduction in accommodative movements than does the ciliary body. Together, this data shows that although age changes occur in the accommodative structures outside of the lens, the age changes that occur in the primate lens contribute significantly to the progression of presbyopia.